Engine wise, there is not much similarity between the last full-house GP250 Honda (2009 RSW) and the last production RS, the NXA.
The RSW had different crankcases, cylinders, PV's, pistons, heads, ECU / PGM, carbs, pipes - see pic.

I was told by an informed source that the last factory 250's (Aprillia) were making around 107 BHP rwhp.
Aoyama's 2009 Honda was not far off them on top end.

For the 98 RS, Honda claim 87 PS, or 86 BHP, and this will be at the crank not rear wheel.
I'd expect maximum of mid 70's BHP at the rear wheel (varying on jetting). Obviously the RS can be tuned significantly.

When comparing to your RG500, is your bike tuned? Standard RG's gave around 80 - 85 BHP at the rear wheel.

But anyway, think about power to weight.
Your RG 500 (tuned?), rear wheel power 100 BHP, dry weight around 155 kg, so p/w = 0.645 BHP per kg.
98 RS, dry weight around 102 kg, so p/w = 0.735 BHP per kg.

If your RG is producing 100 BHP at rear wheel, it may well top out at a higher terminal speed than an RS but not by much, but the RS is going to get to it's terminal speed much faster.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30